Solar fuels A new thin-film coating made from titanium dioxide could convert sunlight to a zero-emission fuel more efficiently.

The findings, from a paper in this week's issue of Science, bring the dream of artificial photosynthesis one step closer, says author Nathan Lewis, a chemistry professor, specialising in solar fuels at Caltech.

Solar panels convert sunlight into usable energy, but a major aim of sustainable energy researchers is to convert sunlight into storable chemical fuels such as hydrogen.

Plants already convert solar into chemical energy using the process of photosynthesis, but this natural process is very inefficient, says Lewis.

"Less than 1 per cent of the energy in the sunlight that strikes the plant is stored in the biomass of the plant," he says.

"So we're trying to build systems that are 10 times more efficient, but that are also robust, last a long time and are cost effective."

The aim would be to have solar fuel generators on rooftops and in fields producing liquid or gas fuels for cars, buildings and industry.

However, artificial photosynthesis has so far met with "limited success", says Lewis.

Weak point

A typical solar fuel generator not only reduces water to produce hydrogen fuel but it also oxidises water to produce oxygen, which can be used to burn the fuel, producing more water.

A semiconductor called a photoanode is required to make the oxygen but this has tended to be the weak point in creating a solar fuel generator, says Lewis.

"Oxidising water to oxygen has been very problematic because almost all common semiconductor materials rust, so instead of oxidising water they oxidise themselves," he says.

Lewis says there have been various attempts to coat the photoanode to protect it from rusting, but so far these have either not been successful at stopping corrosion or have stopped electricity flowing through the photoanode altogether.

"We've figured a way around that, at least on a laboratory timescale," says Lewis.

Thin-film coating

In research funded by the US Department of Energy, he and colleagues have created a special thin-film coating of amorphous titanium dioxide that ensures the photoanode selectively oxidises water.

The titanium dioxide coating is an electrically-conductive version of a chemical used in white paint or sunscreen.

"It is deposited layer by layer using a process called atomic layer deposition," says Lewis.

The coating allows electricity to flow through the photoanode, and at the same time protects it from corrosion.

Previous attempts to coat photoanodes have only produced hydrogen and oxygen safely for a matter of seconds, says Lewis.

By contrast, he and colleagues have run their artificial photosynthesis system for 100 hours of continuous simulated sunlight, with just a 10 percent decrease in the production of oxygen and hydrogen over that time.

Lewis says further research is required into, for example, how the system can fail.

But, he says, it is promising that the coating works not only on silicon but other common semiconductors including gallium arsenide and gallium phosphide.

"The nice thing is it's not material specific," says Lewis.

He says this means many materials that have previously been rejected for development because they were unstable can now be reconsidered for solar fuel generators.